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A thermocouple gauges temperature by utilizing a pair of wires crafted from dissimilar metals. The two wires are interconnected at one end, often through the process of welding. The distinct thermoelectric properties of these wires generate a minute electrical potential between their unattached ends, which can be used to determine the temperature of the interconnected points.
No external power source is necessary for a thermocouple, yet the generated voltage is exceptionally small (measured in microvolts, not just millivolts) and exhibits nonlinearity. Consequently, specialized hardware and/or software are essential to convert this voltage into an accurate temperature reading. To fulfill this requirement, a variety of options are available, ranging from dedicated laboratory equipment to integrated circuit chips.
Thermocouples come in different types tailored to measure various temperature ranges. Each type possesses its own unique characteristics, demanding the use of appropriate conversion methods to derive meaningful temperature values. Moreover, advancements in thermocouple technology have led to the development of innovative features and functionalities to enhance temperature sensing capabilities. These advancements include improved accuracy, faster response times, and greater compatibility with modern instrumentation systems.
Thermocouples or thermocouples are formed when two dissimilar metals come together to form a junction. An electrical circuit is completed by joining the other ends of the dissimilar metals to form a second junction. A current will flow in the circuit if the two junctions are at different temperatures, as shown in next figure.
Figure 1: Closeup of the welded wires in a K-type thermocouple. The background grid is in millimeters.
A commercially available thermocouple is typically enclosed within a probe, as illustrated in the accompanying diagram.
Figure 2: A probe that contains a thermocouple.
Figure 3. depicts a commonly employed schematic symbol utilized to represent a thermocouple. It is worth noting that this particular component does not require an electric current to function. Therefore, the plus and minus signs within the symbol do not indicate the application of power to the wires. Instead, the positive sign signifies the wire that will produce a higher voltage compared to the wire denoted by the negative sign. This distinction aids in determining the polarity of the generated voltage.
Applications of Thermocouples
Thermocouples are highly versatile temperature sensors, offering a broader temperature range compared to other contact-based alternatives. Certain thermocouple types can accurately measure temperatures as high as 1,800 degrees Celsius. However, it is important to consider the joint connecting the wires as it must be capable of withstanding extreme heat. Adequate insulation is crucial, and if necessary, ceramic tube segments are available specifically designed for this purpose.
One of the notable advantages of thermocouples is their minimal thermal mass, which enables them to swiftly respond to temperature fluctuations. Moreover, they do not experience self-heating as they do not consume any power. Their simplicity and robustness make them reliable in various applications. Nevertheless, it is important to note that their response exhibits significant nonlinearity, and the small voltages they generate can be susceptible to electrical noise interference. Consequently, the achievable accuracy of thermocouples typically ranges within plus or minus 0.5 degrees Celsius, with lower accuracy expected at lower temperatures.
Thermocouples find wide usage in laboratory settings and are commonly employed in industrial applications such as monitoring temperatures in blast furnaces or within internal combustion engines. They are even capable of measuring extremely low temperatures down to -200 degrees Celsius. However, it should be noted that at temperatures below -100 degrees Celsius, the temperature coefficient diminishes, resulting in voltage increments of less than 30µV per degree Celsius. This characteristic needs to be taken into account when utilizing thermocouples in low-temperature scenarios.
Working Principle of a Thermocouple
When there is a temperature difference between the two ends of a wire, a temperature gradient is established along the wire. This gradient induces a small electromotive force, which manifests as a variation in electrical potential between the two ends. This phenomenon is known as the Seebeck effect, named after the scientist who first discovered it. The magnitude of the potential depends on two key factors: the temperature difference between the wire ends and the type of wire used.
To illustrate this concept, refer to Figure 4. In Part 1 of the figure, two wires labeled A and B are shown. The left ends of both wires are heated to the same temperature, TX, while the right ends are maintained at a cooler temperature, TY. Due to the wires being composed of different metals, there will be a discrepancy in the voltage drop across each wire.
In Part 2 of the figure, the hot ends of the wires are welded together. This ensures that they share the same temperature and voltage, denoted as VX. However, the exact values of VX are still unknown.
In Part 3 of the figure, the cold ends of the wires are clamped within an isothermal block, maintaining them at an equal temperature (TY). Since the block is not electrically conductive, the cold ends of the wires still exhibit different voltages, VA and VB. Direct measurement of these voltages is not feasible since they are relative to VX, which is unknown. However, a voltmeter can measure the voltage difference (VM) between VA and VB.
The voltmeter itself may have voltage and temperature gradients along its wires, but both wires are made of the same metal, such as copper, and share the same temperature gradient. Consequently, their effects cancel each other out. A mathematical relationship exists between the temperature gradient and the voltage difference in each wire of the thermocouple. Let KA represent a constant or function that relates the voltage difference in wire A to its temperature gradient, and similarly, KB for wire B. Let TDIF denote the temperature difference between TX and TY. We can express it as follows:
KA * (TDIF) = VX - VA
KB * (TDIF) = VX - VB
By subtracting the second equation from the first and rearranging the terms, we obtain:
TDIF * (KA - KB) = VX - VA - VX + VB
The two VX terms cancel out, leaving VB - VA on the right side. We can designate VB - VA as the voltage difference measured by the meter (VM). Therefore: TDIF = VM / (KA - KB)
This equation enables the calculation of the temperature difference between the wire ends based on the meter reading and the conversion factor for each wire, which can be determined experimentally. Since TY is held at a known constant value, the value of TX can be determined as:
TX = TY + TDIF
This methodology allows for the determination of the temperature difference and absolute temperature at the heated end of the thermocouple wire, employing the principles of the Seebeck effect and voltage measurement.
In the early days of thermocouple development, the cold ends of the wires were immersed in an ice and water bath, ensuring a constant temperature of 0 degrees Celsius. However, with the advent of accurately calibrated thermistors, it became possible to directly measure the temperature of the cold ends. This integration of a thermistor with a thermocouple facilitated its functioning. This raises the question: why not solely rely on the thermistor to measure TX and discard the thermocouple? The reason is that thermistors have a more limited temperature range, typically not exceeding 150 degrees Celsius.
It is important to note that the "hot end" of the thermocouple wires does not necessarily have to be hotter than the "cold end," despite the common usage of these terms. The equation used to determine TX remains valid even if TY is higher than TX. In such cases, the temperature difference will have a negative value instead of a positive one.
Recognizing that the terms "hot" and "cold" can be misleading, modern documentation commonly refers to the "measurement junction" and the "reference junction" of the wires. However, it should be noted that the wires are not physically joined at the reference junction.
One prevalent misconception is that voltage is generated at the point where the wires are joined at the measurement junction. This is not accurate. The voltage is actually a result of the temperature gradient between the measurement junction and the reference junction in each wire. Therefore, the specific method used to join the wires is irrelevant as long as there is an electrical connection between them. They can be welded, soldered, brazed, or even crimped together.
By understanding these nuances, a comprehensive understanding of thermocouples and their operation can be gained.
Utilizing a Thermocouple
In laboratory settings, when employing a thermocouple, it is customary to insulate each wire individually, which culminates in a plug that can be inserted into a meter. Within the meter, the reference junction remains concealed, accompanied by electronic components responsible for deciphering the temperature data. To ensure accurate readings, the meter must be set to the appropriate configuration specific to the type of thermocouple being utilized, ensuring the correct conversion factors are applied.
Maintaining consistency in the metal composition of the wires is crucial from the measurement junction to the reference junction. Consequently, it is not feasible to extend the reach of a thermocouple using alternative wire types. Any extensions required must be implemented using wires composed of the same metals as the original thermocouple. Furthermore, connectors employed should have pins and sockets designed to match the specific metal types present in the wires, ensuring compatibility and reliable connectivity.
Varieties of Thermocouples
Thermocouples are classified using ANSI standard codes, employing single letters from the alphabet. The following list provides an overview of these codes along with their approximate temperature ranges, given in Celsius. Please note that practical applications may recommend narrower temperature ranges for optimal performance.
K type: Temperature range: -250 to +1,350 degrees Celsius. This type is highly popular and widely used, particularly in 3D printers. The positive wire consists of a nickel-chromium alloy, while the negative wire is composed of a nickel-aluminum alloy.
J type: Temperature range: -200 to +1,200 degrees Celsius. Featuring an iron positive wire and a copper-nickel negative wire, this thermocouple is magnetic and susceptible to corrosion. It is not recommended for low-temperature measurements, despite its theoretical capability.
T type: Temperature range: -250 to +400 degrees Celsius. Ideal for cryogenic applications, this thermocouple employs a copper positive wire and a copper-nickel negative wire.
E type: Temperature range: -250 to +1,000 degrees Celsius. Known for its high sensitivity and temperature coefficient, the E type thermocouple utilizes a nickel-chromium alloy as the positive wire and a copper-nickel alloy as the negative wire.
N type: Temperature range: -250 to +1,300 degrees Celsius. Serving as an alternative to the K type, the N type is more stable at higher temperatures. It incorporates a nickel-chromium-silicon alloy as the positive wire and a nickel-silicon-magnesium alloy as the negative wire.
R type: Temperature range: -50 to +1,750 degrees Celsius. Designed for high-temperature applications, the R type thermocouple employs a platinum-rhodium alloy as the positive wire and a platinum wire as the negative wire. It exhibits a very low temperature coefficient.
S type: Temperature range: -50 to +1,750 degrees Celsius. Similar to the R type, the S type thermocouple is suitable for high-temperature environments. It utilizes a platinum-rhodium alloy for both the positive and negative wires, offering a very low temperature coefficient.
The Peltier effect states that if a current flows through a thermocouple, one junction heats up (producing energy) while the other junction cools down (absorbing energy). The Peltier effect is a thermoelectric phenomenon related to heat transfer between two different conducting materials when an electric current passes through the junction of these materials. It was discovered by the French physicist Jean Charles Athanase Peltier in 1834.
When an electric current flows through the junction of two different conducting materials, heat transfer occurs. If the electric current flows in a specific direction, heat is absorbed from one side of the junction and released on the other side, creating a temperature gradient. This phenomenon is known as the Peltier effect.
The Peltier effect is based on the interaction of electrons with the atoms of the conducting materials. When electrons pass from one material to another at the junction, they undergo a change in their energy level. This energy change results in heat transfer. The material through which electrons flow from the cold side to the hot side cools down, while the material through which electrons flow from the hot side to the cold side heats up.
The Peltier effect has several practical applications. It is used in thermoelectric cooling devices such as portable food coolers, electronic refrigerators, and cooling systems in space applications. These devices harness the Peltier effect to transfer heat from one side to another using electric current as the energy source.
Moreover, the Peltier effect is also utilized in thermoelectric power generation. By applying a temperature gradient to a thermoelectric device, an electric current can be generated. This electric current is produced due to the reverse Seebeck effect, which is the reverse of the Peltier effect. Thermoelectric power generation devices are used in applications where it is necessary to harness waste heat, such as in power plants or industrial heat recovery systems.
In summary, the Peltier effect describes the heat transfer that occurs when an electric current passes through the junction of two different conducting materials. It is a phenomenon utilized in thermoelectric cooling devices and thermoelectric power generation.
Thompson Effect The Thompson effect states that when an electric current flows through a conductor with a temperature gradient, heat is either produced or absorbed, depending on the direction of the current and the temperature variation. The Thompson effect, also known as the Joule-Thomson effect, is a physical phenomenon that describes the temperature change of a gas when it undergoes adiabatic expansion or compression, meaning without any heat exchange with its surroundings.
When a gas undergoes adiabatic expansion, meaning it expands without heat being added or extracted from the system, it experiences a temperature change. The Thompson effect states that the gas can either cool down or heat up, depending on its properties and conditions.
If the gas behaves as an ideal gas and shows no significant interactions between its molecules, the temperature change during adiabatic expansion is primarily due to the attractive or repulsive forces between the gas molecules. When a gas expands in a region where intermolecular forces are predominantly attractive, the gas cools down. On the other hand, if the gas expands in a region where intermolecular forces are predominantly repulsive, the gas heats up.
The Thompson effect is utilized in various fields and applications. Some of them include:
Refrigeration: The Thompson effect is harnessed in gas-based refrigeration systems. By allowing a gas to undergo adiabatic expansion in a low-pressure zone, the gas is cooled. This is used in food refrigeration, air conditioning systems, and gas coolers.
Oil and Gas Industry: In the production and transportation of oil and gas, the Thompson effect is relevant for determining temperature and pressure changes in fluids during expansion and flow in pipelines and equipment.
Scientific Research: The Thompson effect is studied in scientific research to gain a better understanding of gas behavior and molecular interactions. This is important in the design of cooling systems and in understanding thermal phenomena and heat transfer.
In summary, the Thompson effect describes the temperature change of a gas during adiabatic expansion or compression. It can result in either cooling or heating of the gas, depending on the intermolecular forces present. This effect has applications in refrigeration, the oil and gas industry, and scientific research.